The present invention relates to solid polymer electrolyte fuel cells and in particular, to an improved flow path in an oxidant flow channel of the fuel cell.
A fuel cell generates electricity from an electrochemical reaction between a fuel and an oxidant. The primary components of a fuel cell are an electrolyte sandwiched by a pair of electrodes, namely an anode and cathode, fuel delivery means to the anode, and oxidant delivery means to the cathode. An example of such a fuel cell is a solid polymer electrolyte membrane (PEM) type fuel cell that generates electricity using hydrogen as a fuel and oxygen as an oxidant. A PEM fuel cell includes a membrane electrode assembly (MEA) comprising two gas diffusion electrodes (GDE) and a solid polymer ion-exchange membrane electrolyte in between the electrodes. The membrane material permits the transmission through the membrane of hydrogen ions, but is substantially impervious to the passage of hydrogen or oxygen molecules. Each of the electrodes is coated on one side with a thin catalyst layer. Sandwiching the MEA are a pair of fluid flow field separator plates. Typically, hydrogen fuel is supplied to the anode through one or more fuel flow paths formed between a fuel flow field separator plate and the anode side of the MEA layer; oxidant is supplied to the cathode through one or more air flow paths formed between an air flow field separator plate and the cathode side of the MEA layer. A coolant plate circulating cooling fluid may be positioned adjacent to one or both flow field separator plates to remove heat generated as a byproduct of the electrochemical reaction. Alternatively, cooling channels may be incorporated into the body of one or more flow field plates.
In each fuel cell, hydrogen fuel is oxidized to generate into free electrons and protons (that is, hydrogen ions) in the presence of the catalyst at the anode. The electrons are conducted through a circuit, creating a current of useful electricity usable by a load connected to the circuit. The hydrogen protons migrate through the membrane electrolyte to the cathode. At the cathode, oxygen from the air, electrons from the circuit, and the protons combine to complete the electrochemical reaction, forming water and heat as byproducts. Multiple fuel cells may be stacked together to multiply the amount of electricity generated during operation.
Fuel and oxidant are transmitted through the flow field plates through one or more respective fuel and oxidant flow channels between the MEA layer and the respective fuel or air flow field plate. In typical conventional fuel cells, at least one open-faced channel is formed on a major surface of the flow field plate, typically by machining, moulding or printing. The open channel has a floor at the bottom of the channel and side walls; these surfaces are typically smooth, but may have some minor surface irregularities. The top open face of the channel is covered by the MEA layer when the fuel cell is assembled, thereby enclosing the channel and enabling fluid to flow therethrough. The channels are typically of rectangular shape in cross-section, and extend across the flow field plate in a variety of configurations, such as straight parallel pathways or, one or more serpentine pathways. Each end of each channel is connected to a fluid supply inlet and fluid discharge outlet of the separator plate, respectively, and reactant fluid may be fed in and out of the flow field plate by external or internal manifolding.
Conventional flow field plates are typically made by machining a suitable electrically conductive material, such as graphite. Manufacturing flow field plates out of graphite is desirable as graphite is suitably rigid, gas-impermeable, chemically inert, and relatively inexpensive. Alternatively, flow field plates may be manufactured by a screen printing technique that deposits liquid-formable layers of ink onto a substrate, as disclosed in British Patent Publication No. 2 336 712 A (British Patent Application No. 9909214.0 published Oct. 27, 1999).
Industry has recognized that one of the limiting factors to the performance of fuel cells using air as an oxidant carrier (in contrast to fuel cells using pure oxygen) is providing sufficient oxygen in the air flow channel to sustain the desired rate of the electrochemical reaction. For conventional fuel cell structures, it has been found that increasing the rate of oxygen supplied to the fuel cell relative to the supply of hydrogen will generally increase the reaction rate. Known means of increasing the oxygen supply include operating the air pumps at a setting that provides a sufficiently large concentration gradient between oxygen and hydrogen, that is, maintaining a sufficiently large air-fuel stoichiometry ratio to provide the desired reaction rate. Other known means include using filters or similar devices to extract nitrogen from the air stream prior to reaction, thereby effectively enriching the oxygen content in the air stream.
These known means tend either to be expensive, or impose a substantial load on the electrical circuit that significantly reduces the net power density producible by the fuel cell, or both.
Preferably, fuel cell system efficiency and performance are improved without substantially increasing manufacturing costs. In particular, it is desired to reduce the parasitic losses in a fuel cell system to improve performance and efficiency, either by increasing the power output producible for a given air-to-fuel stoichiometry ratio, or by decreasing the air-to-fuel stoichiometry ratio required for a given power output.
For solid polymer electrolyte membrane (PEM) fuel cells operating on dilute oxidant streams (for example, air), a significant impediment to efficient operation may be the depletion of oxygen in the layer of dilute oxidant nearest the cathode, as a consequence of laminar flow through the air flow path. The formation of an oxygen-depleted layer at the cathode imposes mass transport limitations on the electrochemical reaction, thereby limiting the electric power that can be generated by the electrochemical reaction. During fuel cell operation, oxygen tends to be extracted from the part of the oxidant stream nearest the cathode-air interface. For a substantially laminar air flow, a layer of oxygen-depleted oxidant tends to form and linger in the vicinity of the cathode-air interface during fuel cell operation. The relative scarcity of oxygen molecules near the cathode-air interface results in a relatively low oxygen concentration gradient across the electrode layer that limits the electrochemical reaction rate, and ultimately, the fuel cell performance.
An oxidant flow field plate is provided for a PEM fuel cell that has a membrane electrode assembly (MEA) layer in adjacent contiguous contact with the flow field plate so as to provide mechanical and electrical continuity. The flow field plate includes at least one open-faced oxidant flow channel formed in a major surface of the flow field plate. When the fuel cell is assembled, the open face of the oxidant flow channel is covered by the MEA layer, so that the channel forms a conduit for transmitting dilute oxidant. The oxidant flow channel or channels thus constitute an oxidant flow path for delivery of oxygen to the MEA layer. The structure so far described is conventional.
At least one obstacle for disrupting laminar flow of diluted oxidant passing through the channel is disposed in the oxidant channel. The dimensions of the obstacle and its position in the channel are selected such that when a laminar oxidant flow passing through the channel encounters the obstacle, the laminar flow pattern in the vicinity of the obstacle and especially at the MEA-oxidant interface is disrupted and changed to a turbulent flow pattern. Such disruption mixes dilute oxidant relatively lean in oxygen near the MEA-air interface with dilute oxidant relatively rich in oxygen elsewhere in the channel, effectively increasing the amount of oxygen near the MEA-oxidant interface available for electrochemical reaction. In other words, the oxygen concentration near the MEA-oxidant interface is substantially reconstituted as a result of the turbulent, twisting or otherwise non-laminar flow patterns generated by the obstacle, so as to make available at the MEA layer oxygen that, were it not for the obstacle, would have tended to pass by the MEA layer in a more remote part of the laminar flow.
The channel may be formed with opposed side walls and a floor. In one aspect of the improved oxidant flow field, the obstacle protrudes from the floor and extends between the side walls. The obstacle may be of various shapes, dimensions, and positioned in various places within the flow channel; several obstacles may be positioned in sequence to maintain turbulent flow. Each obstacle should interfere minimally with oxidant flow rate, thus implying that the longitudinal dimension (that is, the dimension in the direction of the oxidant flow) should be relatively short. Since any constriction of the flow channel negatively affects flow rate, the constriction of the flow channel created by the obstacle should be the minimum to create the desired turbulence.
A suitable obstacle is a thin slab whose plane of orientation is transverse relative to the oxidant flow direction. The laminar character of the oxidant flow striking the upstream major surface of the slab is disrupted; the diluted oxidant is forced into turbulent flow as it crosses over the exposed edge of the slab.
Alternatively, the obstacle may be positioned in the channel such that its major surface facing generally upstream is angled relative to the transverse dimension of the channel. The angling causes an oxidant stream encountering the obstacle to twist, thereby causing the oxidant stream to invert and further contributing to the mixing of the diluted oxidant stream. The optimal angle for creating a suitable twisting pattern depends on a number of parameters, including channel dimensions, obstacle dimensions, oxidant flow rate, oxidant temperature, and the like. Useful twisting patterns have been generated by obstacles positioned at angles between xe2x88x9265xc2x0 to +65xc2x0 relative to the transverse dimension of the channel.
In accordance with another aspect of the improved oxidant flow field, there is provided an obstacle having a pair of protrusions extending from the channel floor. Each protrusion has a width of about half the width of the channel, and is transversely positioned in the channel such that together, the protrusions span the width of the channel. The protrusions are longitudinally positioned offset from each other in the channel. In one embodiment, the protrusions are thin planar slabs; the slabs have their major planar surface transversely oriented; that is, they are positioned in the channel such that the major surface faces the direction of oxidant flow. Each slab is offset in the longitudinal channel direction a selected distance from the other slab in order to cause an oxidant flow encountering each slab to form a twisting and turbulent flow pattern, thereby mixing the oxidant in the channel in the vicinity of the slabs, and especially at the MEA-oxidant interface. Alternatively, either slab or both slabs may be angled relative to the oxidant flow at an angle other than 90xc2x0. In another embodiment, each protrusion is ramp-shaped, and is positioned in the channel such that one ramp faces upstream and the other ramp faces downstream, and such that the top edge of each ramp is transversely in-line with the other. This forces oxidant encountering the ramp to twist and become turbulent, thereby mixing the diluted air stream in the vicinity of the ramps, and especially at the MEA-channel interface.
In accordance with yet another aspect of the improved oxidant flow field, there is provided a helical obstacle for insertion into an oxidant flow channel. The helical obstacle is positioned in the channel such that its axis is generally parallel to the longitudinal dimension of the channel. Such an obstacle encourages oxidant flowing by the obstacle to form a twisting and turbulent pattern that mixes the dilute oxidant stream in the channel in the vicinity of the obstacle, and especially at the MEA-channel interface. For example, the helical obstacle may be a coil. The helical obstacle has a width approximately equal to the channel width, and the obstacle may be secured inside by an interference fit in the channel, adhesive or other suitable means.
The channel surface for each of the above described embodiments of the improved oxidant flow field may optionally be textured to further agitate the oxidant flowing near the channel surfaces. Such channel wall texture may be provided by a plurality of embossments and/or depressions along the channel walls.
Except for obstacles like the aforementioned helical obstacles, which are inserted into an associated channel, the obstacles are preferably integrally formed with the flow field plate by, for example, one of machining, molding or printing.
There are competing design considerations in the choice of obstacle design for a fuel cell oxidant path; an empirical approach is recommended to maximize the benefit that can be obtained by the improved oxidant flow field under different operating conditions. For example, obstacles inherently impede oxidant flow and thus lower flow rate; the contribution provided by the obstacles in promoting turbulent oxidant flow should be balanced against the necessity to maintain an adequate flow rate and acceptable pressure drop. Obstacles should not be positioned in such a way as unduly to impede access of oxidant to the MEA layer; this objective tends to limit the number of obstacles present and constrains the location and orientation of obstacles. Also, the presence of obstacles tends to add to manufacturing expense. Rough-textured oxidant channel walls may generate some localized turbulence in the oxidant flow, although rough walls per se would not be expected to be sufficient to provide a suitably well mixed oxidant stream. A design balance should be sought and achieved between these and possibly other factors.